Titanium is an example of one of the metals which has found widespread use as an electrically conductive layer in semiconductor microelectronic circuits. Titanium is used in microelectronic devices to provide low-resistance electrical contact to materials, such as silicon or aluminum, which form a stable insulating oxide layer on their surface. Titanium also serves as a bonding agent between materials, such as silicon dioxide and tungsten, which otherwise do not bond strongly together.
As microelectronics manufacturers have attempted to make devices that operate faster and are less expensive, they have made narrower openings through which the metallic connections must pass through the insulating layers. In microelectronics the "aspect ratio" is defined as the ratio of the thickness of the insulator layer to the diameter of a hole or the width of a trench through the insulator. In current practice, aspect ratios of 1 or 2 are commonly used. In the next generation of devices, it is widely believed in the industry that aspect ratios of 3 or 4 or more will be used.
Sputtering is usually used to form the titanium and titanium nitride layers in computer processors, memories and other microcircuits. Although sputtering has been successful in coating devices with currently used aspect ratios, it is difficult for sputtering to coat uniformly devices with higher aspect ratios. Sputtering forms coatings which are thicker at the top surface and thinner on the bottoms and lower side walls of holes and trenches, and therefore, it is said that sputtering has poor "step coverage". While this difficulty of sputtering can be alleviated somewhat by collimating the sputtered material, this leads to other difficulties including poor sidewall coverage and high cost because of reduced coating rate and greater maintenance requirements.
An alternative coating process with good step coverage would thus be highly useful in semiconductor manufacture. Chemical vapor deposition processes sometimes show very good step coverage, and for this reason a CVD process for titanium having good step coverage, operating at low enough temperatures, and having relatively non-corrosive byproducts would be advantageous in the manufacture of semiconductor microcircuits.
The requirement for low temperatures is particularly important. In modern semiconductor designs several layers of metal interconnections are applied and titanium layers are often used to enhance electrical contact between these successive metal layers. Temperatures during the formation of these upper layers of metallization must be kept below about 400.degree. C. in order to avoid thermal degradation of these structures. Unfortunately, there are no prior art CVD processes for depositing titanium that meet all these requirements.
There have been a number of attempts to form titanium by chemical vapor deposition from a number of different reactants. The reaction of titanium tetrahalides with molecular hydrogen is spontaneous only at very high temperatures, which would cause degradation of silicon semiconductor structures. Another difficulty with using this process for semiconductors is that some halogen is deposited as an impurity in the titanium. This residual halogen may cause corrosion of the metal layers. It may also be corrosive to the apparatus used, so that expensive materials of construction must be used.
A lower temperature CVD process for titanium is disclosed in German patent 1,117,964 (Nov. 23, 1961). This process involves the thermal decomposition of vapors of dicyclopentadienyl titanium at temperatures of 260.degree. to 482.2.degree. C. A similar process for depositing titanium, zirconium or hafnium has been proposed in European patent publication 0 468 396 A1 (Jul. 22, 1991) using compounds such as cycloheptatrienyl cyclopentadienyl titanium.
Silicon, a widely used semiconductor, is deposited from a variety of CVD sources, particularly silane, disilane and dichlorosilane, which are hazardous spontaneously flammable gases. It would be advantageous to have CVD sources for silicon which are not as hazardous as these gases.
Boron and phosphorus are added to silicon as impurities (called dopants) to increase its electrical conductivity. Diborane and phosphine are extremely poisonous gases which are used to provide dopants during CVD of silicon or silicon dioxide. It would be advantageous to have less toxic boron and phosphorus sources which also have lower vapor pressures, so that they could not be spread as easily by gas leaks.
Gallium arsenide is another useful semiconductor which is often made by a CVD process starting from arsine, an extremely toxic gas, and trimethylgallium, which is spontaneously flammable. It would be desirable to have less toxic arsenic sources with lower vapor pressures, and gallium sources which are not spontaneously flammable.
Aluminum is a metal that is useful in making interconnections in microcircuits. Currently known CVD aluminum sources include aluminum alkyls, which are spontaneously flammable and also leave carbon impurities in the aluminum, and aluminum hydride complexes, which are unstable when stored at room temperature. Because of these disadvantages, aluminum is not usually made by CVD. It would be desirable to have aluminum CVD sources which make pure metal, are not spontaneously flammable and are stable during storage.